Propylene impact copolymers (“ICP's”) are used in materials handling applications such as pallets and crates, where good creep resistance and impact properties are required. Creep resistance is the property of deformation under load conditions. The creep resistance is proportional to the stiffness (e.g., Flexural Modulus) of the product and inversely related to the product melt flow rate (“MFR”, 2.16 kg, 230° C.). Namely, the creep is lower with higher stiffness product, and is lower with lower MFR (or high molecular weight) product.
However, the cycle time—the time requiring to mold a single part by injection molding—is high with low MFR products (e.g., less than 4 dg/min) and thus it is preferred to have higher MFR product to achieve lower cycle time. In addition, impact resistance also needs to be adequate for the parts used under different temperature conditions, and the impact resistance in impact copolymers is a function of the ethylene-propylene rubber content. This is especially true for articles such as pallets and crates having a surface contacting face and a load carrying face with a space there between for accepting a carrier such as a fork lift, etc. Both surfaces must be strong enough to carry loads of 500 to 1200 lbs or more at temperatures as low as −10° C., yet the composition used to make the injection molded pallet must have an adequately high MFR to allow rapid cycle time production.
For improved cycle time with ICP's, high MFR product is desirable as it is much easier to fill out the mold in comparison to a low MFR product. However, the high MFR ICP product requires even higher MFR polypropylene matrix in comparison to a low MFR ICP. For example, a 10 dg/min ICP, with about 18 wt % ethylene-propylene (“EP”) rubber requires a matrix polypropylene having an MFR of approximately 20 dg/min versus only about 8 dg/min polypropylene for a 4 dg/min ICP. Due to the much lower polypropylene MFR in the 4 MFR ICP as compared to the 10 MFR ICP, the creep resistance is significantly better with the 4 MFR ICP.
Therefore, there is a desire to achieve the delicate balance between ICP MFR for better cycle time and creep resistance for use under load conditions, and the required impact resistance. The inventors solve this and other problems by providing a composition and method of producing an ICP having at least two polypropylenes having unbalanced MFRs.